The present invention is generally drawn to boilers using an economizer to transfer waste heat in flue gas to boiler feedwater and employing Selective Catalytic Reduction (SCR) reactors to remove NOx from the flue gas, and, more particularly, to the optimized temperature operation of same over a variable load range.
In fossil-fuel fired boiler systems economizers perform a key function in providing high overall boiler thermal efficiency by recovering the low level, i.e. low temperature, energy from the flue gas. Economizers recover the energy by heating the boiler feedwater, thereby cooling the flue gas. For each 40 degrees F. (22 degrees C.) that the flue gas exit stack temperature is cooled the overall boiler efficiency increases by approximately 1%.
Economizers are typically tubular heat transfer surfaces used to preheat boiler feedwater supplied to the boiler. As shown in FIG. 1, a common economizer design uses bare, in-line, serpentine tubes with the flue gas flowing vertically downward in a cross counter-flow heat exchange relationship with boiler feedwater 150 flowing upwardly through the tubes. Due to the relatively small difference between the temperature of the flue gas and the temperature of the boiler feedwater, economizers require a large amount of heat transfer surface per unit of heat recovered. In some applications, fins may be applied to the outside of the tubes to improve the controlling gas side heat transfer rate. The economizer is generally the last water-cooled heat transfer surface upstream of an air heater, a gas-gas heat exchanger used to preheat the combustion air.
SCR reactors are used to reduce impurities from the flue gases, or exhaust gases, of boiler and furnaces, and in particular, to reduce NOx emissions. Ammonia or an ammonia precursor is injected into the boiler flue gas stream in the presence of a catalyst. Chemical reactions occur with the flue gas, which removes a large portion of NOx from the flue gas and converts it to water and elemental nitrogen. The SCR reactions take place within a required temperature range. Most can operate within a range of 450 to 840 degrees F. but optimum performance occurs between 500 to 750 degrees F. Outside of the recommended temperature range, many catalyst materials become less effective or fail to perform the intended function. Further, flue gases containing sulfur oxides are further restricted to lower limit temperatures from 600 to 650 degrees F. to avoid degrading the performance of a downstream air pre-heater.
Additional details of SCR systems for NOx removal are provided in Chapter 35 of Steam/its generation and use, 40th Edition, Stultz and Kitto, Eds., Copyright ©1992, The Babcock & Wilcox Company, the text of which is hereby incorporated by reference as though fully set forth herein.
Since SCR reactions take place within a required temperature range, the SCR reactors are typically located downstream of the economizer flue gas outlet of a steam generator or boiler and upstream (with respect to a direction of flue gas flow) of any air heater devices used to preheat the incoming combustion air.
For economic reasons the desired gas temperature entering the SCR reactor should be maintained in the required range at all loads, from full load down to partial loads. Also, maintaining the desired flue gas temperature reduces the formation of ammonia and/or sulfate salts within or on the ammonia injection system and the catalyst. However, as boiler load decreases, the boiler exit gas temperature will drop below the optimal temperature range. To increase the gas temperature to the required temperature range while minimizing the impact on full load thermal efficiency, current practice has been to use an economizer gas bypass flue 80, shown in FIG. 2. The economizer gas bypass flue 80 is used to remove some of the hotter flue gases upstream of the economizer, and then recombine the hotter flue gas with cooler flue gas that leaves the economizer thereby raising the overall flue gas temperature. By controlling the amount of gas that flows through the bypass system, the flue gas temperature entering the SCR reactor can be maintained within the required temperature range at the lower boiler loads.
In another approach to dealing with decreasing flue gas temperature entering an SCR reactor at reduced boiler loads, an economizer was fitted with a modulated partial feedwater bypass to maintain the flue gas temperature at low load without reducing full load thermal efficiency.
Both of the above approaches for mitigating the effects of boiler load changes on the operation of an SCR reactor 20 are active methods requiring the use of valves, dampers or other shut-off means, such as damper 94 shown in FIG. 2.
Retrofit applications of SCR systems to steam generators having limited space present their own particular problems.
The size of the catalyst bed required to achieve effective NOx reduction at a utility power generation station is very, very large. For ease in handling and installation, the blocks are fabricated into large modules. For example, an SCR system built by The Babcock & Wilcox Company and retrofit to a 675 MW coal-fired power station included 31,664 cubic feet (897 cubic meters) of 0.25 in. (6 mm), plate-type catalyst. Such large catalyst arrangements, with their related installation and system modification requirements, are expensive to build.
A sectional side view of one such installation is shown in FIG. 2. In this conventional configuration, SCR reactor 20 of the SCR system includes several catalyst layers 30. Flue gas is discharged from SCR reactor 20 into an existing air heater 60. The SCR system is designed with downflow of the flue gas, after upflow ductwork for an ammonia injection system 10 and mixing. This results in a vertical reactor at a high elevation. As a consequence, construction costs represent a substantial total of the cost of an SCR system, particularly for retrofit systems. With as much as 50% of the capital cost of an SCR retrofit involving construction of the equipment, constructability is thus an important design consideration for cost reduction. While existing structural steel 50 may be used, the FIG. 2 shows that a large amount of new structural steel 40 is needed to bear the weight of the SCR system, and the associated upstream and downstream flues. The foundation for the SCR system and structural steel must also be taken into consideration, and may require modification for retrofit installations.
Increasingly stringent environmental regulations continue to place pressures upon electric utilities which use fossil-fueled boilers or steam generators to produce electricity. However, modifications to existing boilers are often problematic due to the limited space available, and the utilities' desire to make such modifications in an efficient manner and at minimum cost. Thus improvements that allow for more economic installation and operation of SCR reactors for boiler flue gas cleanup would be welcomed by industry.